Process for preparing a multiviscosity lubricating oil



United States Patent Ofiice q 3,011,974 Patented Dec. 5, 1961- 3,011,974 PROCESS FOR PREPARING A MULTI- VISCOSITY LUBRICATING OIL Alfred M. Henke, Springdale, William A. Home, Oakrnont, and Harry C. Staufier, Cheswick, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Filed June 22, 1959, Ser. No. 821,649 6 Claims. (Cl. 208308) This invention relates to the preparation of multiviscosity lubricating oils and more particularly to a novel method of preparing rnulti-viscosity lubricating oils through the use of liquid thermal diffusion.

Mineral lubricating oils known as multi-viscosity, multigrade or double branded lubricating oils are lubricating oils capable of meeting two or more viscosity grade specifications of the Society of Automotive Engineers (SAE). Such oils are desirable for use as crankcase oils when it is necessary to have a lubricant of sufficiently low viscosity at low temperature to permit easy engine starting and of suficiently high viscosity at higher temperature to provide satisfactory lubrication at normal engine temperatures.

At the present time, multi-viscosity oils are prepared by adding certm'n synthetic organic compounds known as viscosity index'improvers to a lubricating oil of conventional viscosity specifications in an amount sufiicient to meet multi-grade specifications. Typical viscosity'index improvers are high molecular weight polymers such as butene polymers, polymers of the esters of methacrylic acid and higher fatty alcohols, mkyl styrene polymers, etc. For conventional multi-grade lubricating oils the viscosity index improvers are added to the oil in amounts up to 8 to 10 percent by weight of the oil. This adds substantially to the cost of the product. Furthermore, many of the known viscosity index improve'rs are objectionable because of instability. Oils containing such additives may deteriorate when vigorously agitated or when subjected to high shear rates and stresses, as occurs in lubri cation service. Oils containing large concentrations of the polymer-type viscosity index improvers appear to behave as non-Newtonian fluids when subjected to high shear rates and tend to approach the viscosity of the original mineral lubricating oil.

We have now discovered that successful rn-ulti-viscosity lubricating oils can be produced without the addition of large amounts of the objectionable synthetic viscosity index improvers. We have discovered that multi-viscosity lubricating oils with viscosity indices of at least 132, such as oils corresponding to the SAE specifications for a W/20 or a W/ multi-viscosity oil, can be prepared without synthetic viscosity index improving additives by our novel procedure of subjecting selected mineral lubrieating oil fractions to liquid thermal diffusion and blending the products obtained thereby.

The phenomenon known as thermal diffusion occurs when a temperature gradient is established across a fluid between the hot and cold parts of the mixture. Thermal i mixture and causes differences in concentration to develop 60 dilfusion can be put to practical advantage in the separation of liquid mixtures. It is known to subject lubricating oils to liquid thermal diffusion to obtain a fraction thereof of high quality. An apparatus for this purpose comprises two concentric vertical tubes that form smooth, impervious walls of an annular chamber or thermal diffusion slit. One of the walls is heated and the other is cooled. The distance between the hot and the cold walls is very small and the temperature difference per unit of distance across the thermal diffusion slit is very high.

The liquid mixture to be separated is introduced into the annular chamber or slit. Certain components tend to diffuse toward and concentrate along the hot wall while other components tend to diffuse toward and concentrate along the cold wall. The differences in temperature and liquid density result in convection currents and cause the liquid along the hot wall to rise and the liquid along the cold wall to flow downwardly. From the top of the thermal diffusion slit a fraction of the original mixture enriched in components that diffuse toward the hot wall is recovered and frorn the bottom a fraction enriched in components'that diffuse toward the cold wall is recovered. In subjecting mineral lubricating oils to thermal'diifusion it has been found that the top product or the fraction enriched'in components that tend to diffuse toward the three or more viscosity grades, for example, the 5W/20.

and 10W/ 30 grades (also designated as SW-SAE 20 and 10W-SAE 30), are very severe with respect to viscosity- 7 temperature characteristics of the oils. This is illustrated by the following table which indicates the SAE viscosity specifications for SW/ZO and l0W/30 oils. The

table lists the required viscosities for the different grades in Saybolt Universal seconds (hereinafter abbreviated as SUS) as determined by ASTM Method D-88.

TABLE I Viscosity'values for crankcase oils Viscosity Range, Saybolt Universal Seconds sAE viscos t 1 I Number At 0 F. At 210 F.

Minimum Maximum Minimum Maximum Minimum viscosity at 0 F. may be waived rovided viscosit at 210 F. is not below 40 SUS. p y

210 F. is not below SUS.

Lubricant-oil compositions thatcover only {we sAE r 3 ing three or more viscosity grades and that have viscosity indices of 132 or higher such as the SW/ 20 and 10W/ 30 oils which have viscosity-temperature characteristics as indicated in Table 1.

Despite the fact that it has been known that liquid thermal diffusion can be used to separate from a lubricating oil a fraction of higher viscosity index than the original oil, heretofore it has been necessary to add large amounts of expensive synthetic additives to such products to make a lubricant conforming to severe multi-grade specifications. Although oils of high viscosity index have been made by thermal diffusion of certain lubricating oil fractions, to the best of our knowledge such oils have invariably been either too viscous at F. or too thin at 210 F. to meet severe multi-grade specifications without adding to them various synthetic viscosity index improvers. We have now developed a new method by means of which it is possible to make lubricating oil compositions which meet severe multi-grade viscosity specifications, e.g., SW-SAE 20 V and l0W-SAE 30, and which consist essentially of mineral oil components.

Our method in general comprises subjecting to liquid thermal diffusion a selected light petroleum lubricating oil and a selected heavy petroleum lubricating oil and blending the hot wall fractions or high viscosity index fractions thereof to obtain a product which meets the viscosity requirements for a multi-viscosity lubricating oil. Alternatively, the method comprises blending the selected light and heavy petroleum lubricating oils, thermally diffusing the blend and recovering a hot wall fraction thereof which meets the viscosity requirements for a multiviscosity lubricating oil. The light oil feed stock is a light mineral lubricating oil which has no more than a low content of wax and of materials of aromatic character and which has a viscosity index of at least 85 and a viscosity at 210 F. of 40 to 50 SUS. The heavy oil feed stock is a heavy mineral lubricating oil which has no more than a low content of wax and of materials of aromatic character and which has a viscosity index of at least 85 and a viscosity at 210 F. of 90 to 130 SUS. The variables for liquid thermal diffusion of the light oil, including the feed rate, the width and length of the thermal diifusion slit and the temperature difference across the slit are selected to produce a hot wall fraction of the light oil amounting to 30 to 60 volume percent of the light oil and having a viscosity index of at least 135. The variables for thermal diffusion of the heavy oil are selected to produce a hot wall fraction thereof amounting to 30 to 60 volume percent of the heavy oil and having a viscosity index of at least 115.. The hot Wall fractions obtained by thermal .dittusion of the light and heavy oils are blended in a ratio to form a multi-viscosity oil having a viscosity index of at least 132 when the oil contains no synthetic viscosity index improving additives. In the alternative method, the blend of the light and heavy oils is thermally diifused under conditions to produce a hot wall fraction in an amount of 30 to 60 volume percent of the blend having a viscosity index of at least 132. e

We will describe our invention in more detail by reference to particular examples of preparations in accordance with the invention.

EXAMPLE 1 The starting materials were a premium grade light petroleum lubricating oil of SAE 'classification and a premium grade heavy petroleum lubricating oil of SAE 50 classification. The SAE .10 oil was prepared by subjecting a paraflin base Ordovician crude to atmospheric and vacuum distillation to obtain a light lubricating oil fraction. This fraction was subjected to a number of lubricating oil refining procedures including solvent refining by furfural extraction, solvent dewaxing using methylethylketone-toluene as the solvent, treatment with aluminum chloride and clay treating. The heavy oil was obtained from a heavy lubricating oil fraction of the same parafiin base cr de and was subjected to the same refining procedures as the light oil, with the exception that the solvent refining procedure was phenol extraction, and in addition the oil was subjected to propane deasphalting. Both of the base oils as employed as feed stocks in this example were straight lubricating oils and contained-no synthetic additives. The SAE 10 base oil had a gravity of 31.8 API, a viscosity at 210 F., of 44.6 SUS, a viscosity index of 110, a pour point of 10 F. and an average molecular weight of 400. The SAE 50 base oil had a gravity of 279 API, a viscosity at 210 F. of 97.4 SUS, a viscosity index of 102, a pour point of 0 F. and an average molecular weight of 618. The described light and heavy oils were separately subjected to continuous-flow, liquid thermal diffusion. The thermal diffusion apparatuswas a vertical concentric tube column which was 12 feet in length and had a slit width of 0.032 inch. The thermal diffusion slit was filled with oil and the oil was charged at a feed rate of 12 ml. per hour. A hot wall temperature of 345 F. and an average temperature of F. for the cold wall coolant water were maintained. The rate of withdrawal of bottoms product in relation to feed rate was controlled to provide equal volume yields of overhead and bottoms fractions from the column. The overhead or hot wall fraction of the light oil was blended with the hot wall fraction of the heavy oil in a volume ratio of 7 '1 percent light oil component to 29 percent heavy oil component to form a blended lubricating oil meeting the viscosity requirements for 5W/ 20 oils without the use of synthetic additives. Inspections of the feed stocks and of the product of this example, which we designate as Oil A, are given in Table II hereinafter.

EXAMPLE 2 To prepare an oil meeting the viscosity requirements for 10W/ 30 oils, the SAE 10 light oil component is thermally diffused in the same manner described in Example 1 and the SAE 50 heavy oil component is thermally diffused in the same manner as in Example 1, except that the feed rate is lower, namely, 6 ml. per hour instead of 12 ml. per hour. This will result in a 50 percent overhead fraction of the heavy oil of higher viscosity index than was obtained in Example 1 and is necessary in order to meet the more severe specifications for a 10W/30 oil when the heavy fraction is blended with the light fraction. Inspection data for a 50 percent overhead fraction of the SAE 50 oil obtainable as described in this example and of the blend, designated as Oil B, which can be obtained by blending the described overhead fraction of the SAE 50 oil with the overhead fraction of the SAE 10 oil of Eaxmple 1 are given in Table II hereinafter.

Although the procedure of Examples 1 and 2, in which the light and heavy oils are separately thermally diffused and in which the products thereof are then blended is our preferred procedure and yields higher quality products, as we have indicated a possible modification of our process comprises blending the light and heavy base oils and then thermally diffusing the blend. This procedure in accordance with the invention is described in the following example.

EXAMPLE 3 The SAE 10 and SAE 50 base oils described in Example 1 were blended in a ratio of 68 volume percent of the light oil to 32 volume percent of the heavy oil. The blend was then subjected to liquid thermal diffusion in the manner described in Example 1. The feed rate of blended oil to the thermal diffusion column was 25 ml. per hour. The hot wall temperature was 340 F. The average coolant water temperature for the cold wall was 105 F. and the overhead product amounted to 50 volume percent of the charge. The overhead fraction, which we designate as Oil C, was an oil of high viscosity index which substantially meets the requirements for a 5W/20 oil and is comparable in quality to the oil obtained by blending the thermally ditfused product of Example 1. Inspections of this product are listed in Table H.

TABLE II Inspections charge stocks and products Feed Stocks Thermal Difiusion Overhead Products Example 1 Example 2 Example SAE 10 SAE 50 Oil Oil 50% 50% 50% Oil 0 Ovhd. of Ovhd. of 011 A Ovhd. of Oil B SAE 10 SAE 50 SAE 50 Blend, Vol. Percent SAE 10 Oil Compon 71 12 68 Gravity, API 31.8 27. 9 36 4 31. 2 35. 0 32. 3 32.8 34. 7 Viscosity, SUS:

At 0 F. (Extrapolated) 8, 900 185, 000 2, 125 28, 000 4, 000 18, 000 12, 000 4, 200 At 100 F 163 1, 110 142 3' 298 147 At 210 F 44. 6 97. 4 40. 4 67. 5 45. 0 62 58. 0 45.0 V scosity Index 110 102 145 1.20 139 120 132 133 Four Point, F 0 35 20 20 From the above table it can be seen that the 50 percent overhead fraction of the SAE 10 oil obtained in Example 1 had a much higher viscosity index than the thermal diffusion feed stock. Specifically, the overhead fraction had a viscosity index of 145 as compared with 110 for the SAE 10 oil. However, despite the great improvement in the viscosity index, the thermally diffused product was far short of the viscosity requirements for a multi-viscosity oil. Specifically, the viscosity at 210 F. was only 40.4 SUS as compared with the minimum viscosity at 210 F. of 45 SUS for an SAE 5W/20 oil and 58.0 SUS for an SAE 10W/ oil. Although the numerical difierence betwwn the viscosity of the oil at 210 F. and the required viscosity for the multi-viscosity oils appears small, such a difference in this viscosity range represents a major difference in the quality of the oil in view of the logarithmic relationship between viscosity and temperature. Thus, thermal diifusion of the single oil stock, SAE 10, was unsuccessful in making a product meeting'the severe multiviscosity specifications.

Table II shows that thermal difiusion of the SAE 50 oil was equally unsucessful in producing an oil meeting severe multi-viscosity requirements. Thus, the 50 percent overhead fraction of the SAE 50 oil obtained in Example 1 had a higher viscosity index than the SAE 50 base oil, specifically 120 as compared with 102. But the viscosity of the 50 percent thermal diflusion overhead fraction at 0 F. (extrapolated) was 28,000 SUS which greatly exceeds the maximum of 4,000 SUS at 0 F. for a 5W/20 oil and the maximum of 12,000 SUS at 0 F. for a l-OW/ 30 oil.

The advantages of our invention are illustrated by the inspections of Oils A, B and C of Examples 1, 2 and 3 as shown in Table II. Oil A is a blend comprising 71 volume percent of the 50 percent thermal diffusion overhead or hot wall fraction of the SAE 10 base oil and 29 volume percent of the 50 percent thermal difiusion overhead fraction of the SAE 50 base oil as obtained in Example 1. The blended oil meets the viscosity requirements for a 5W/20 multi-viscosity oil. The blended oil had a pour point of 20 F. However, the pour point can be lowered to 10 F. or lower by adding a small amount, for example, 0.5 weight percent of a conventional pour point depressant.

Oil B of Example 2 will meet the viscosity requirements for a 10W/ 30 multi-viscosity oil. This oil is a blend of the 50 percent thermal diffusion overhead fractions of the SAE 10 oil of Example 1 and of the SAE 50 oil of Example 2 in volume proportions of 12 parts of the light fraction per 88 parts of the heavy fraction. The Example 2 overhead fraction of the SAE 50 oil is obtained at a lower feed rate than the corresponding fraction of Example l and has a higher viscosity index. This higher quality heavy fraction is used for Oil B in order to meet the more severe requirements of a 10W/ 30 oil. The resulting blended Oil B will have a viscosity at 210 F. of 58 SUS and a viscosity at 0 F. of 12,000 SUS which meet the requirements for a l0W/30 oil. The viscosity index of Oil B will be 132 and the pour point about 20 F. By adding a small amount, e.g., 0.5 volume percent, of a conventional pour point depressant the pour point of Oil B can be lowered to a satisfactory level.

Oil C of Example 3 which was prepared by blending the SAE 10 and SAE 50 .base oils and subjecting the blend to thermal diffusion to obtain a 50 percent overhead or hot wall fraction of high viscosity index, had a viscosity index of 133 and a viscosity of 45.0 SUS at 210 P. which meets the minimum viscosity at 210 F. for an SAE SW/ZO oil.' The viscosity at 0 F. (extrapolated) was 4,200 SUS as compared with the maximum of 4,000 SUS at 0 F. for an SAE SW/ZO oil. The pour point of Oil C was 25 F. However, as we have already indicated, a small amount of a conventional pour point depressant will lower this pour point satisfactorily. Furthermore, by using a pour point depressant such as Acryloid, a high molecular weight polymerization product of the esters of methacrylic acid and higher fatty alcohols such as cetyl or lauryl, which also functions as a viscosity index improver, the viscosity index of Oil C can be raised sufiiciently to form an oil that meets the viscosity requirements for a 5W/20 oil. The difierence between the viscosity of Oil C at 0 F. of about 4,200 SUS and the maximum of 4,000 SUS at 0 F. for a 5W/20 oil does not represent a great difference in the character of the oil. A small amount of the mentioned pour point depressant, e.g., less than 0.5 volume percent, will normally improve the viscosity index of Oil C sufiiciently to meet the SW/ 20 specifications. Also, by adjusting the thermal difiusion conditions and/or blending proportions as used in the example it is possible to obtain an oil which will meet the 5W/20 specifications without addition of synthetic viscosity index improving additives. For example, by increasing the temperature gradient across the thermal diilusion slit or by lowering the feed space velocity we can obtain an overhead fraction of higher viscosity index than Oil C which will meet the 5W/20 specifications.

We have described the use of specific light and heavy lubricating oil feed stocks for preparing our blended and thermally difiused oils. Although it is essential in accordance with our invention to employ selected light and heavy oils as starting materials, the invention is not limited to the use of the specific oils described in the examples. The light oil feed stock is a light lubricating oil having a viscosity index of at least and a viscosity at 210 F. of 40 to 50 SUS, and is obtained by atmospheric and vacuum distillation of a paraffin base crude oil which is capable of producing a high yield of high quality light lubricating oil. The light oil should have no more than a low content of wax and of materials of aromatic character. If the quality of the straight run light lubricating oil fraction is adequate, little or no additional refining will be necessary to prepare it as a feed stock for our process but normally the light oil will be solvent-refined refining procedures such as furfural extraction for re-' moving aromatics from lubricating oil fractions and any of the conventional dewaxing procedures, such as solvent dewaxing with a methylethylketone-toluene solvent, can be employed.

V The heavy oil feed stock for our process must have a viscosity index of at least 85 and a viscosity at 210 F. of 90 to 130 SUS. The oil is obtained by atmospheric and vacuum distillation of a' parafi'in-base crude to obtain a high yield of heavy lubricating oil of high viscosity index. The heavy lubricating oil is subjected, as necessary, to refining procedures such as deasphalting, solventrefining and dewaxing to produce a heavy lubricating oil having no more than a low content of wax and materials of aromatic character. Any of the conventional solventrefining processes capable of producing a heavy lubricatdewaxing procedure such as solvent dewaxing to produce a heavy lubricating oil having a pour point no higher than about 10 F.

It is possible in our process to obtain products of lower pour points than indicated for Oils A, B and C by close control of the temperature during dewaxing'of the light and heavy :oil feed stocks. We have demonstrated this in dewaxing a partially refined oil to prepare a thermal diffusion feed stock. Thus, we have solvent dewaxed an Ordovician unpressable distillate using ketone as the solvent at a 10:1 solvent to oil ratio and at three dewaxing temperatures, specifically, 15 F., l F. and 35 F. The dewaxed oils were subjected to thermal diffusion substantially as described in Example 1 to obtain an overhead fi-action amounting to about 30 volume percent of the charge. The effect of the dewaxing temperature for the thermal diffusion feed stock on the pour point and viscosity index of the thermal diffusion overhead product is shown in the following table.

The above data show that there is a relationship between the temperature at which the solvent dewaxing is carried out and the pour point of the dewaxed oil after thermal difiusion. The overhead products from the thermal difiusion column had lower pour points at lower solvent dewaxing temperatures. However, the viscosity index was also decreased at the 30 F. dewaxing temperature of run 3. Therefore, the thermal diffusion feed stock should be dewaxed at a sufiiciently low temperature so that the feed stock will yield a thermal diffusion overhead product having a pour point in the range from about -25 F. to F. With such a product pour point depressants can either be eliminated or used only in very small amounts and the product will have a sufliciently high viscosity index to form the ultimate multi-viscosity blended oil products of our process. As indicated by Table III the dewaxing temperature should not be so low as to greatly reduce the viscosity index of the thermal diffusion overhead fraction.

We have described specific conditions and apparatus for thermal diffusion of lubricating oils. However, the conditions and apparatus can be varied considerably. The variables in thermal diffusion separation include the slit width and length, hot wall temperature, cold wall temperature, feed rate and the ratio of top product or hot wall product to bottom product or cold wall product.

The temperatures for the hot wall and the cold wall are selected to provide a large temperature gradient across the thermal diffusion slit because the rate of separation increases as the difference in temperature per unit of distance across the slit increases. However, temperature must be chosen that will provide a reasonably large gradient across the slit without either wall temperature being so low as to make the liquid too viscous or so high as to cause the liquid to decompose or vaporize. In thermal difiusion hot and cold are relative terms. Both the hot and cold walls may be above or below atmospheric temperature. In fractionating lubricating oils in the range of SAE 10 to SAE 50 a hot wall temperature in the range of 200 to 500 F. and a cold wall temperature in the range of to F. give a suitable balance of the different factors. 7

The slit width is an important variable in thermal diffusion. The. slit must be narrow so that a high temperature gradient per unit of distance will be obtained but if it is too narrow, the capacity of the unit and hence the maximum feed rate for satisfactory fractionation will be too low. A slit width from about 0.01 to 0.2 inch is used in the method of our invention for fractionating light and heavy lubricating oils.

The feed rate or space velocity of the lubricating oil subjected to thermal diifusion depends upon the degree of fractionation that is desired. By feed rate or space velocity we mean the volume of feed charged per volume of slit capacity per hour. In separating lubricating oils of the SAE 10 to SAE 50 range we use space velocities of about 0.05 to 0.2 liquid volumes per volume of slit capacity per hour, depending upon the results desired. Lower space velocity can be used if very careful fractionation is desired and faster rates when the degree of fractionation does not have to be so high. As is usual in liquid thermal diffusion operations the feed rate must be low enough to provide non-turbulent flow of liquid in the thermal diifusion slit as turbulent flow will interfere with the diffusion of components toward the hot and cold walls of the apparatus and with the convection currents which cause fractions to migrate toward one end or the other of the apparatus.

Although a vertical, concentric tube type of apparatus was used in our examples, we can also use a parallel plate type of apparatus in which the thermal diffusion slit is bounded by two substantially parallel fiat plates. Furthermore, the apparatus is not necessarily vertical but can be horizontal or inclined at any suitable angle. In a horizontal apparatus it will generally be necessary to provide some moving element to perform the function that is performed by convection currents in the vertical type of apparatus to cause migration of the hot wall product to one end of the slit and of the cold wall product to the other end of the slit. In one apparatus of this type either the hot or the cold wall is in the form of a moving endless belt. Although the examples of this specification describe continuous thermal diffusion separation, it is within the scope of our invention to employ batch type thermal diffusion separations.

With respect to the ratio of top to bottom product or hot wall product to cold wall product, it can be said in general that the highest quality hot wall product is obtained with the lowest yield. In other words, if the ratio of hot wall product to cold wall product is small the hot wall fraction of a lubricating oil will have a very high viscosity index. However, it is usually economically impracticable to operate with very low yields of hot wall or top product. An equal yield of top and bottom product is preferred in our process and for satisfactory results the hot wall fraction can range from 30 to 60 volume percent of the feed. A

Still another variable in our method of preparing'multiviscosity oils is the variation in proportions of the light and heavy oil components of the product. Thus, the light and heavy oils can be blended in various proportions prior to thermal difiusion and in the alternative method in which the light and heavy oils are thermally difiused before blending, the proportions of the thermally diffused light and heavy oils in the final blend can be varied. A precise definition of the ratios of the light and heavy oil fractions cannot be given because the ratios will depend upon the characteristics desired for the resulting blend. Thus, in preparing the relatively heavy SAE 10W/3O oil the ratio of the heavy oil fraction to the light oil fraction will be greater than for a blend intended to meet the specifications for the lighter SAE W/20 oil. In general, in forming our multi-viscosity oils the blend of thermal diffusion hot wall fractions or the blend of light and heavy oils to be subjected to thermal diffusion will contain from about to 70 volume percent of the light oil component.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are in dicated in the appended claims.

We claim:

1. A method for making a multi-viscosity mineral lubricating oil which comprises subjecting to liquid thermal diiTusion a light mineral lubricating oil having a viscosity index of at least 85 and a viscosity at 210 F. of 40 to 50 SUS, recovering a hot wall fraction thereof amounting to 30 to 60 volume percent of said light oil having a viscosity index of at least 135, subjecting to liquid thermal diffusion a relatively heavy mineral lubricating oil having a viscosity index of at least 85 and a viscosity at 210 F. of 90 to 130 SUS, recovering a hot wall fraction thereof amounting to 30 to 60 volume percent or" said heavy oil having a viscosity index of at least 115, and blending one of said hot Wall fractions into the other of said hot wall fractions until a multiviscosity lubricating oil having a viscosity index of at least 132 is formed.

2. The method according to claim 1 in which one of said hot Wall fractions is blended into the other of said hot wall fractions until a 5W/20 multi-viscosity lubricating oil having a viscosity at 0 P. not higher than 4,000 SUS and a viscosity at 210 P. not lower than 45 SUS is formed.

3. The method according to claim 1 in which one of said hot wall fractions is blended into the other of said hot Wall fractions until a 1OW/30 multi-viscosity lubricating oil having a viscosity at 0 P. not higher than 12,000 SUS and a viscosity at 210 F. not lower than 58 SUS is formed.

4. A method for making a multi-viscosity lubricating oil which comprises blending a light mineral lubricating oil having a viscosity index of at least 85 and a viscosity at 210 F. of 40 to SUS with a heavy mineral lubricating oil having a viscosity index of at least and a viscosity at 210 F. of to SUS until the resulting blend contains from 10 to 70 volume percent of said light oil, subjecting the blend to liquid thermal ditfusion, and recovering a hot wall fraction of the blend amounting to 30 to 60 volume percent thereof, said fraction having a viscosity index of at least 132 and meeting the viscosity requirements for a multi-viscosity lubricating oil.

5. A method according to claim 4 for making a SW/ 20 'multi-viscosity lubricating oil in which said light and heavy oils are blended until the resulting blend contains from 10 to 70 volume percent of said light oil and in which the blend is subjected to liquid thermal difiusion to produce a 30 to 60 volume percent hot wal fraction having a viscosity at 0 P. not higher than 4,000 SUS and a viscosity at 210 F. not lower than 45 SUS.

6. A method according to claim 4 for making a 10W/ 30 multi-viscosity lubricating oil in which said light and heavy oils are blended until the resulting blend contains from 10 to 70 volume percent of said light oil and in which the blend is subjected to liquid thermal diffusion to produce a 30 to 60 volume percent hot wall fraction having a viscosity at 0 P. not higher than 12,000 SUS and a viscosity at 210 P. not lower than 58 SUS.

References Cited in the file of this patent UNITED STATES PATENTS 2,077,781 Story Apr. 20, 1937 2,084,512 Swoope et a1 June 22, 1937 2,183,783 Bray Dec. 19, 1939 2,339,898 White et a1. Jan. 25, 1944 2,360,446 Reid Oct. 17, 1944 2,541,070 Jones et a1. Feb. 13, 1951 2,853,427 Bentley et al. Sept. 23, 1958 

1. A METHOD FOR MAKING A MULTI-VISCOSITY MINERAL LUBRICATING OIL WHICH COMPRISES SUBJECTING TO LIQUID THERMAL DIFFUSION A LIGHT MINERAL LUBRICATING OIL HAVING A VISCOSITY INDEX OF AT LEAST 85 AND A VISCOSITY AT 210* F. OF 40 TO 50 SUS, RECOVERING A HOT WALL FRACTION THEREOF AMOUNTING TO 30 TO 60 VOLUME PERCENT OF SAID LIGHT OIL HAVING A VISCOSITY INDEX OF AT LEAST 135, SUBJECTING TO LIQUID THERMAL DIFFUSION A RELATIVELY HEAVY MINERAL LUBRICATING OIL HAVING A VISCOSITY INDEX OF AT LEAST 85 AND A VISCOSITY AT 210*F. OF 90 TO 130 SUS, RECOVERING A HOT WALL FRACTION THEREOF AMOUNTING TO 30 TO 60 VOLUME PERCENT OF SAID HEAVY OIL HAVING A VISCOSITY INDEX OF AT LEAST 115, AND BLENDING ONE OF SAID HOT WALL FRACTIONS INTO THE OTHER OF SAID HOT WALL FRACTIONS UNTIL A MULTIVISCOSITY LUBRICATING OIL HAVING A VISCOSITY INDEX OF AT LEAST 132 IS FORMED. 